Std 8 General Science · Chapter 4

4. Current Electricity and Magnetism

Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Eight General Science.

Exercises

1Write proper words from the following group of words in the blanks. (magnetism, 4.5V, 3.0V, gravitational attraction, potential difference, potential, higher, lower, 0V)

(a) Water in the waterfall flows from a higher level to the lower level because of gravitational attraction.

(b) In an electric circuit, electrons flow from a point of lower potential to the point of higher potential.

(c) The difference between the electrostatic potential of the positive end and the negative end of an electric cell is the potential difference of the cell.

(d) Three electric cells of potential difference 1.5 V each have been connected as a battery. The potential difference of the battery will be 4.5 V.

(e) An electric current flowing in a wire creates magnetism around the wire.

2A battery is to be formed by joining 3 dry cells with connecting wires. Show how will you connect the wires by drawing a diagram.

The positive terminal of one cell is joined to the negative terminal of the next cell. This is called series connection.

A battery of three dry cells connected in series.

3In an electric circuit, a battery and a bulb have been connected and the battery consists of two cells of equal potential difference. If the bulb is not glowing, then which tests will you perform in order to find out the reason for the bulb not glowing?

Check whether the switch is on.
Check whether all connections are tight.
Check whether any wire is broken.
Check whether the positive terminal of one cell is joined to the negative terminal of the other.
Check whether the cells are discharged.
Check whether the filament of the bulb is broken.

4Electric cells having 2V potential difference each have been connected in the form of a battery. What will be the total potential difference of the battery in both cases?

In series, the total potential difference is the sum of the potential differences of all cells.

(i) 3 cells: 2 V + 2 V + 2 V = 6 V
(ii) 4 cells: 2 V + 2 V + 2 V + 2 V = 8 V

5Describe the construction, working and usefulness of a dry cell, with the help of a diagram.

Construction of a dry cell.

Construction:

The outer container is made of zinc. It is the negative terminal.
A graphite rod with a brass cap is at the centre. It is the positive terminal.
A paste of manganese dioxide surrounds the graphite rod.
Between them is a paste of ammonium chloride and zinc chloride. It acts as the electrolyte.
The top is sealed.

Working:

Chemical reactions take place in the cell. Positive charge collects on the graphite rod and negative charge on the zinc container. A potential difference of 1.5 V is produced. When the terminals are connected, current flows in the circuit.

Usefulness:

Dry cells are used in torches, radios, wall clocks, remote controls and toys. They are easy to carry and can be used in any direction.

6Describe the construction and working of an electric bell with the help of a diagram.

Construction of an electric bell.

Construction:

An insulated copper wire is wound around an iron core. It works as an electromagnet.
A soft iron strip with a striker is fixed near the electromagnet on a spring.
A gong is fixed near the striker.
A contact screw touches the iron strip.
The coil, strip, screw, battery and switch are connected in a circuit.

Working:

When the switch is pressed, current flows. The iron core becomes a magnet.
The magnet attracts the iron strip. The striker hits the gong and the bell rings.
Now the strip leaves the contact screw. The circuit breaks and the magnetism is lost.
The spring pulls the strip back. It touches the screw again and the circuit is complete.
This repeats again and again. So the bell rings continuously.

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